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How do I know which outer grid shape is best for CFD?
I’m looking into running a CFD simulation on an aerofoil in transonic flow (M=0.8) but am unsure about what external grid shape to choose for my computational domain?
I read into 3 main types:
C-grid
Elliptical
Circular
How do you choose a grid? How does it affect the simulation and why would this even have an impact on the simulation please?

These grid topologies refer to structured meshes.  Unstructured meshes are a different technology with different pros/cons, I won't discuss them here.
A mesh must have adequate resolution to resolve all the flow features of interest using the numerical methods of the chosen tool.
Low quality cells can adversely impact a solution.  For structured meshes, this usually means highly non-orthogonal (or skew) meshes.  It can also have to do with how quickly the mesh resolution changes in a given direction.
2D Structured meshes can be represented by a matrix of points.  X(I,J) and Y(I,J).  You can easily move from one point to the next by indexing in the I or J direction -- i.e. move from XY(I,J) to XY(I+1,J).  If I starts at 1 and runs to N, then those values of indices are valid I[1,N] no matter the J.  Likewise, if J[1,M], those values are always valid.  I.e. there are no holes in IJ space, no gaps, etc.
This means that the mesh has NxM points in it.  If you increase the number of points in I, those points exist for all J.  This means that if you need to add points in one region to resolve the flow, you might end up with extra points somewhere else that you don't need.
A very simple topology is the O-grid, you might think of it as a circle grid.

In the O-grid, the I=1 line is the airfoil and the I=N line is the outer boundary.  The J=1 and J=M lines are coincident, and usually are chosen to start at the trailing edge of the airfoil and travel downstream from there.
Another simple topology is the H-grid.  It can also be thought of as two grids together -- a top grid and a bottom grid.

In an H-grid, the I=1 line is the bottom boundary, the I=N is the top.  The airfoil surface is found along I=N/2, with that line being duplicated or split somehow to represent both the top and bottom surfaces.
The left boundary is J=1 and the right boundary is J=M.
A third common topology is the C-grid.

Here, the I=1 line is both the airfoil and also the split line starting from the trailing edge and extending downstream.  The I=N line is the top, left, and bottom of the outer boundary.  The J=1 line is the top half of the vertical boundary and the J=M line is the bottom half of the vertical boundary.
If you are doing a viscous solution, then you typically need a lot of resolution in the boundary layer.  The O and C grids here depict this situation.
With a C grid, when you have a dense boundary layer, you also automatically get a dense wake downstream of the airfoil.  This can be a benefit if you are interested in resolving the wake downstream.
If you imagine using a H grid and resolving a dense boundary layer, you would also end up with a region of very fine cells extending in front of the airfoil.  Those cells are not needed and the solution would not be very efficient.
However, if you use an O-grid for this solution, you will get a good solution near the airfoil, but if you are interested in the wake downstream of the airfoil, your solution will quickly degrade as the cells get very coarse downstream of the trailing edge.
Another common issue is the trailing edge itself.  The airfoil you are considering might have a sharp TE or a blunt TE.  A sharp TE can be very difficult to resolve with high quality cells with an O-grid or H-grid.  However, a C-grid is very amenable to a sharp TE.
However, a blunt TE is very difficult for a C-grid if you can't insert an additional grid to emanate from the TE itself.
Usually a mesh is described as either algebraic or elliptical.  An algebraic grid uses simple algebraic equations to place the points.  Basically, any point can be calculated directly.  An elliptical grid generator sets up an elliptical partial differential equation to arrange the points.  In this way, the all the boundaries influence the exact placement of all the points.  this is more complex and computationally expensive, but it can produce a very smooth and high quality mesh.
All of these images were pulled from a PDF of Numerical Grid Generation.  This book used to be available freely online from a site at Mississippi State University, but I can't find a link to it anymore.

Hi Rob, thanks for the answer. I still feel confused though about the external grid shape- the above answers are really good, but focus on the internal bit too much please?

As long as the outer grid shape is 'far enough' away, then it doesn't really matter.  If you are doing a supersonic simulation, then nothing in front of the Mach cone emanating from the forward most point of the geometry will matter.  If you are doing a subsonic simulation, then you need mesh in all directions.  For 2D simulations, you probably want the boundary 5-30 chord lengths away -- depending on the fidelity you're after and the computational expense you can spare.  You can run a study of changing the far field distance to see how it changes your solution.


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What is the effect of cold temperatures on aircraft gyroscopic instruments?
When it comes to cold temperatures, I know some effects on engine, for example it causes higher viscosity of oil and so higher oil pressure when engine is just started but still cold.
But what is the effect of cold temperatures on Gyroscopic instruments?

I think this would be a huge answer with lots of possibilities and variances, it can't be answered with "X will happen to all" and this question can be more thoroughly researched.

Gyro instruments will have tiny caged ball and roller bearings for moving components. They will normally contain light oil or possibly a low viscosity grease.
Low temps will have some effect, perhaps making gyros spin up a bit slower until the bearings warm up.  But the effect may be too minor to notice. I've never been able to tell when flying in winter.

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Class Echo Control Tower Procedures
Northern Colorado Regional Airport (KFNL) resides within a surface echo. This airport, however, has an operating control tower. When VFR in echo airspace, ATC services are provided but not mandatory. So does that mean I don't technically need to speak with Loveland tower to get cleared to land? I guess it's just confusing that there's a towered airport that's not class D, C, or B. Thank you!

So-- why is it set up that way, a tower with a surface-level Class E circle around it?

14 CFR 91.127 "Operating on or in the vicinity of an airport in Class E airspace" address this scenario. Specifically, 91.127(c) requires two-way comms to be established "prior to 4 nautical miles from the airport, up to and including 2,500 feet AGL."
You do need to speak with NoCo Tower (not "Loveland Tower;" the airport diagram tells you their callsign) and you do need to receive clearance to land.
If you want to get even weirder, 91.126(d) similarly requires two-way comms with towers in Class G airspace.

Interesting.  So, are there any operating towers in Class G airspace?  Have there ever been?

...you've asked that before

NoCo Tower is a field test facility for remote tower operations. It's a contract operation, and only has partial staffing (right now, there's only one controller there), and is closed most of the time.
The "tower" consists of multiple high-resolution cameras that see the movement areas from multiple angles, and the controller is stationed in a temporary building (aptly named the "horse trailer").
The FAA probably saw no need to reclassify the surface area, since Serco (the contractor) was having difficulty staffing the operation from the outset.
Last I heard, Serco was pulling out of the project, which probably means FNL will need to explore other options. The airport certainly warrants having a tower.

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what are the CAA rules on eye surgery?
I've been assessed and hired as cabin crew member by Easyjet in UK, on the 26th april I will have my medical assessment.
2 weeks ago I did a PRK refractive laser surgery for myopia correction. Today I've got another eyes examination that certified that there is not a refractive error anymore, that my visual acuity is better than aviation standards, that i'm not experiencing post-operative complitations or problematic visual effects as ghosting or halos or glares and so on... So actually all my eye functionality are giving great results.
Axa health medical requested me this report to send to CAA to be analysed and decide if I'm fit to fly and procede with my face-to-face medical assessment.
My worries concern not about the eyes visual standards but the TIME.
Could CAA reject me for a matter of time, even if from my eye surgery date to the start of training pass exactly 2 monthes?
Could CAA complain about a full eye report, even if completely successfull, that has been provided after 2 post operative weeks?
I know is trivial but I would be gratefull if someone could help me... Thanks

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How does a normal aircraft exhaust nozzle (the base model) generates a lot noise?
How does a normal aircraft exhaust nozzle (base model) generates noise and, whats the difference between the operation of the normal base model of aircraft exhaust nozzle and the new design of aircraft nozzle with an add of chevrons ?
Is the chevrons effects the aircraft or engine performance in any way ?

Those "chevrons" are vanes that modify the air and exhaust flow (those are turbo-fan- engines in those two images) and create air vortexes that quiet the noise but also create some drag. They are engineered to maximize noise reduction with a minimum of drag.
(popularmechanics.com) Quiet engine air brake.
There is a company called ATA Engineering that is now developing a product called a "quiet engine air brake" that create more drag but is stowable during cruise flight. The R&D work was commissioned by NASA.  They use the same vortex flow concept. Because most of the noise from airliners during approach comes from things that stick out in the airflow, like flaps and spoilers, landing gear doors, etc., this allows the aircraft designer to generate drag to slow down the aircraft while at the same time reducing noise.
Its a pretty nifty concept, if you ask me. Over time I think we will see the design of exhaust systems on jet engines become stranger and stranger! 8)

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Why can't I hang glide in NYC?
I was stopped by NYPD for attempting to hang glide in an uncongested area by Midland Beach, Staten Island. They called their Legal Bureau and came back with "Illegal Navigation" and if I was to continue I would be arrested and my hang glider would be seized. I called the NYPD Aviation Unit to ask about the legality of my attempted flight and told me to call back again later since this was an unusual request. When I called them back they generically stated that it's not legal under FAA regulations within the NYC area, but did not refer to any specific law and suggested I go to Long Island or Jersey.
What is meant by a "congested area of a city/town/settlement" in FAR 103?
If it's not illegal, then it's legal. So if I can prove that it's not, then I should be legally good to go.

You have two points that contradict.  If they said that it was not legal under FAA regulations (I assume you mean regulations since the FAA doesn't create laws) then you should ask why and not assume that it is legal!

Even with documentation, copies of all relevant laws, etc. proving what you are doing is legal, the cops that stop you won't care - they'll simply ticket/arrest you and let the lawyers deal with it.  As the saying goes, you can beat the rap, but you won't beat the ride.

it's an ultralight, right ?

They told you no, but they couldn't point to a law? I guess you could say they... left you hangin'!

@ivanivan, then they can also risk the lawyers dealing with an unlawful arrest suit.

@MichaelHall .. first time i've logged in in years... and boom. timing, huh?  Anyway, it has been proven quite a few times in court that the police aren't responsible for knowing every nuance of the law, what is legal vs. not legal, etc.  If you want some good examples check youtube for the 1st and 2nd Amendment "auditors".

@ivanivan, I wouldn't expect them to know every nuance, but if they threaten arrest they are required to inform you of the specific statute you are violating, are they not?  And if they are unsure they ought to grant you the benefit of the doubt under the presumption of innocence.  I have watched the auditors you speak of, and those who know their rights and act courteously (many do not!) can get a cop to back down.  "Illegal navigation" sounds laughable to me.

There are (at least) two general points to consider. First, hang gliders are ultralights and the regulations in 14 CFR 103 apply to them. 103.17 says:

No person may operate an ultralight vehicle within Class A, Class B,
  Class C, or Class D airspace or within the lateral boundaries of the
  surface area of Class E airspace designated for an airport unless that
  person has prior authorization from the ATC facility having
  jurisdiction over that airspace.

If you look at a sectional chart, most of the airspace over NYC is class B, including to the surface in many places:

In a class B surface area ultralights require ATC's permission to operate; whether or not the area was congested is a separate issue and nothing to do with the airspace class itself.
Second, as a general statement the FAA regulates what happens in the air but not what happens on the ground. States and counties can make their own rules about launching and recovering aircraft, and national parks have their own rules too. In other words, you need to make sure that wherever you fly your hang glider, it's both legal to launch and legal to operate in that airspace.
Now, let's look at Midland Beach, Staten Island specifically. I've marked the approximate location on the chart with a red dot. It's hard to see because the chart is so crowded, but that part of the coast is depicted as a "special conservation area" and there's also a comment that it's part of the Gateway National Recreation Area, which is a national park.
The airspace there is class B from 1500ft upwards, meaning that if you stay below that altitude then you're OK from the airspace point of view. It's true that the FAA discourages flight over parks, but it's mostly about noise (not a concern in a hang glider) and there's no actual regulation anyway.
However, it seems likely that you were launching from a national park and that does require permission from whichever authority runs it. I have no idea whether or not that's what the NYPD were talking about.
(My personal suspicion is that the NYPD officers just decided "that can't be legal" and made up something on the spot. But there are still actual regulations that do apply, whether or not the officers were fully informed/trained on them or not. I also don't know why the NYPD would want to enforce FAA regulations, but that's a topic for another site.)

Good points. I was around Midland Beach, Staten Island which seems to be uncontrolled below 1500. I'll be happy if I even get off the ground. If I took off from the beach, I would have been in violation of NYC Park violation assuming the beach is part of the park. https://www.nycgovparks.org/rules/section-1-04 But if I was to take off from an uncongested area mid/lower Staten Island ungoverned by NYC Park Laws, then I would not be in violation of FAA regulations as long as I'm below 500ft.

@royjr, For the FAA part, you should just contact them. If they don't have a problem with you, they'll tell you (just like they did with this guy) https://www.youtube.com/watch?v=ciioZOhm1Hg&t=608s If the local police is enforcing non-existent laws at the behest of local politically connected residents, that certainly wouldn't be the first time. On our California coast, we also had that problem until our California Coastal Commission nipped it in the bud by imposing its own $11,250 a day fine.  https://www.theguardian.com/us-news/2015/oct/02/california-wealthy-public-beaches-private-security

https://skyvector.com/
Many areas of NYC are controlled area from the surface (SFC) on up, meaning you need to obtain clearance from the FAA before entering the area, either on the radio, or by a telephone call beforehand to enter the area at a specific time and following a specific flight path. I imagine your hang glider is not radio equipped, thus you would need to be calling for clearance before takeoff.
Look at the chart, drag/pan/zoom in to the NYC area, and see if you were in one of those areas. All the yellow areas should be treated as congested.

As has been noted in a comment to a deleted answer-- Part 91 doesn't apply to hang gliders.  Ultralights are specifically exempted from the entirety of Part 91.  So there's no requirement for a "clearance".  However, as another answer has noted, FAR 101.17 imposes a requirement for prior authorization.


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What causes magnetic compass turning errors?
What exactly causes magnetic compass turning errors? I found two completely different and contrasting explanations online and I'm confused as to which one is correct.
Explanation 1: Reference:

 (Starting from 4:10)
Summary: The compass’s North end dips down due to magnetic dip. Then CoG of the compass swings to the other side (South side) so the other half ends up heavier and with more inertia.

So when turning, particularly from a North or South heading, the centrifugal force pulls on the compass’s CoG on the South side and makes the South side swing outward (toward the outside of the turn). This causes compass to lag when turning away from a North heading and to lead when turning away from a South heading.

Explanation 2: Reference: https://drive.google.com/file/d/1sgfdgSFPfw0jtNaH2kUkIgy1aOtDl_tn/view (Page 4)
Summary: Turning errors fundamentally occur because the aircraft is in a banked position, and the compass card is also banked relative to the horizon. The magnetic dip then makes the North end of the compass want to dip downward. In that banked position, dipping downward means the compass turning so that the North end of the magnet points closer to the direction of bank (the side with the aircraft's lowered wing). This causes compass to lag when turning away from a North heading and to lead when turning away from a South heading.

In this image Fv is the vertical component of the total magnetic force, aka the force representing magnetic dip and causing the North end of the magnet to dip downward. F error is the vector present in a bank that makes the compass turn erroneously (the component of Fv that makes the North end of the magnet point closer to the direction of bank).

The difference I noticed between the two explanations is: the first explanation attributes turning errors to centrifugal force pulling on the compass’s CoG, which the second explanation attributes it to the aircraft being in a banked position and the magnetic dip pulling the north end of the compass down.
My question is, which explanation is correct? Could they both be correct and both plays a factor? If you’re in a slip where you’re banked (matches condion in Explanation 2) but not turning (no centrifugal force, no Explanation 1), would magnetic compass turning errors still be a thing? And if you're in a rudder-only severely skidding turn with no bank (centrifugal force/Explanation 1 only and no factors from Explanation 2), would there still be compass turning errors?

(Note: for convenience, this explanation is phrased for the northern hemisphere only.)
The second explanation is the one I was previously familiar with.  It always made sense to me, and is undoubtedly a major factor in compass "lead" and "lag" in turning flight.
Note the following-- regardless of whether the aircraft is slipping ("overbanked" for the turn rate) or skidding ("underbanked" for the turn rate), the compass is free to tilt as needed to align itself with the apparent "felt" direction of down, as indicated by the slip-skid ball.  This suggests that for any given turn rate, on any given heading, we'll see the same tendency for the compass to "lag" or "lead" regardless of whether we are slipping, skidding, or fully coordinated.1
On the other hand, if the compass were designed differently, so that it pivoted on a fixed axle and was not free to tilt side to side in the aircraft's reference frame, then an unbanked skidding turn would create no compass errors, because the compass card would be perfectly horizontal.  A coordinated turn would create the same errors we see with the normal design, and a slipping turn would create greater errors than we see with the normal design.  Also, flying on a constant heading in even a slight wing-down slip would create a compass error, which would be objectionable.2,3
"Explanation 1" seems problematic.  Essentially it is saying that due to magnetic dip, the compass is tilted relative to the "felt" direction of apparent "down" in the aircraft's own reference frame, which on a northerly or southerly heading causes a fore-and-aft mis-alignment between the pivot point (which is the only point where the airplane can actually apply a force to the compass) and the CG of the compass.  Therefore on northerly or southerly headings, in turning flight the centripetal force exerted by the aircraft on the compass is acting at a point located fore or aft of the CG of compass card.  This causes an inertial swing of the compass card which causes it to "lag" on northerly headings and "lead" on southerly headings.
This seems plausible on first read-through.  But how much "tilt" in the compass card do we see when flying due west or east with the wings level in linear, non-turning flight?  Very little.  This suggests that aviation compasses are designed so that the CG of the compass card is low enough, relative to the pivot point, that the compass card tends to stay oriented rather close to the "felt" up-down direction in the aircraft's reference frame (which in linear wings-level flight is also the actual up-down direction relative to the earth), regardless of the effect of magnetic dip.  So the compass card tends not to react to magnetic dip by tilting to any significant degree.  It mainly reacts to magnetic dip by rotating about the pivot point, when the aircraft is on a magnetic heading with a southerly or northerly component in turning flight.
This isn't to say that the effect of "Explanation 1" is completely negligible, but it undoubtedly makes much less of a contribution to compass "lead" and "lag" than "Explanation 2".  See also footnote 2 below, in relation to the fourth paragraph of this answer-- this seems to be a strong argument that "Explanation 1" is not the dominant cause of compass error.
Footnotes:

This paragraph would seem to be true regardless of whether the second explanation or the first explanation is the dominant cause of compass errors, or whether they both contribute about equally.

This paragraph would seem to be true only if the "Explanation 2" is the dominant cause of compass errors.  If "Explanation 1" were the dominant cause of compass errors, then (at least in coordinated flight) they could be mostly eliminated simply by mounting the compass card on a fixed axle so that the compass card was not allowed the compass to tilt at all as seen in the aircraft's own reference frame.  If it were possible to eliminate the compass errors in turning flight this way, it might be worth putting up with the drawbacks posed by such an arrangement in non-turning flight.  (Or there could be two compasses, one optimized for turning flight and one optimized for linear flight!)  The fact that this is not done in actual practice would seem to be a strong argument that "Explanation 1" is not the dominant cause of compass errors.

These issues no doubt need to be considered in the design of electronic heading sensors with no moving parts.  The simplest possible design would be analogous to a mechanical compass that was modified so that it pivoted on a fixed axle and was not free to tilt side to side in the aircraft's reference frame, but the results would be problematic.

Elaborating further on last sentence before footnotes, and Footnotes 2 and 3-- if "Explanation 1" were the only significant cause of compass error, and "Explanation 2" were insignificant, then a simple 1-axis magnetic sensor such as we find on many hand-held GPS receivers for hiking etc would work fine and be free of turning errors when rigidly mounted in the appropriate orientation in an aircraft.  It does not!

(ctd)  Just as a physical compass rotates to a wrong heading due to magnetic dip in turning flight, so to does the "virtual" needle in the magnetic sensor "rotate" to a wrong heading due to magnetic dip whenever the aircraft is banked.  In actual practice these devices are harder not easier to use in turning flight in an actual aircraft, than a traditional physical magnetic compass.  This suggests that inertia-related issues are really not the primary problem.

Would I be correct to understand that turning errors are essentially present entirely dependant upon whether the "felt" direction of down is misaligned with the actual direction of down relative to the Earth? (As that causes the compass to be tilted relative to the Earth). Therefore with the current freely-tilting compass design, if you had an unbanked skidding turn, you would still experience errors because the "felt" direction of down is tilted toward the outside of the turn due to centrifugal force, and the compass card would also be tilted to match the "felt" direction of down.

Would I also be correct to understand that whether the "felt" direction of down is misaligned with the actual direction of down is dependent upon whether a turn rate is present? Therefore regardless of whether you are unbanked, underbanked, or overbanked, if in all three situations you have the same rate of turn at the same airspeed, then
a) the felt direction of down would all be misaligned to the same degree with the actual direction of down,
b) the compass card in all three situations would be tilted to the same degree, and
c) the same degree of compass errors would be experienced?

The one thing I do wonder about is still the situation of being in a side/forward slip with no change of heading at all/zero turn rate. I'm under the assumption that because during a slip there is no continuous acceleration in any direction (other than the initial change of track in the case of a sideslip), you would experience exactly 1 G, pointed straight down. (So the "felt" direction of down is aligned with the actual direction of down.) As such, with a normal compass design, the compass card wouldn't be tilted relative to the earth and during a slip thus no errors are present.

That last comment about slips is entirely based on my assumption that during slips you experience 1G, with the felt direction of down aligned with the Earth's direction of down. I have no idea if this is true.
Essentially I understood from your explanation that the airplane’s bank angle causes no compass error except insofar as it correlates with rate of turn. If there is rate of turn, there is misaligned (as in misaligned with the actual direction of down) G-force ("felt" direction of down), and THAT is the root cause of compass turning errors. Would I be correct in this understanding?

To add, all of those comments and followup questions are assuming that Explanation 2 is the only significant cause of compass errors (that I understand now thanks to your wonderful explanation!)


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What causes magnetic compass acceleration errors?
I was trying to find out what exactly causes magnetic compass acceleration errors, and I found two completely different and contrasting explanations online (both are supported by an equal amount of reliable sources).
Explanation 1:
Reference:

 (starting at 3:28).
Summary: when on a east or west heading (in the northern hemisphere), the compass’s North end dips down due to magnetic dip. Then CoG of the compass swings to the other side (South side) so the other half ends up heavier and with more inertia.

This image represents the compass mounted on a pivot point, with the magnet shown.
So when you change speed, that heavier end (the South end), having more inertia thus more resistance to speed change, swings forwards/backwards depending on deceleration or acceleration, and compass will indicate turn to the North/South.

Acceleration

Deceleration
Explanation 2:
Reference: https://drive.google.com/file/d/1sgfdgSFPfw0jtNaH2kUkIgy1aOtDl_tn/view (page 4-6: page 6 in particular but pages 4-5 provide a backgrounder)
Summary: I’ve also seen some people say that the compass card’s center of gravity is below its pivot point, thus acceleration causes the bottom-heavy compass card to tilt forward (bottom swings back), and deceleration causes the compass card to tilt backward (bottom swings forward).
The magnetic dip then makes the North end of the compass want to dip downward. At that forward/backward tilted angle, dipping downward means the compass turning so that the North end of the magnet is at a lower point. Hence the compass will indicate turn to the North when accelerating and South when decelerating.

The difference I noticed between the two explanations is that as seen from the pics, the first explanation thinks the compass card is tilted sideways as viewed by pilot (North end down) due to magnetic dip. While the second explanation thinks the compass card is tilted forward/back due to compass card CoG being at the bottom (and magnetic dip comes in after to make the compass card turn). So my question is: which explanation is correct? Could they both be correct?

Hi Katrina, I posted an answer, but then found this: does it answer your question: https://aviation.stackexchange.com/a/78871/42636

@Jpe61 and Katrina -- I'd suggest that that answer is a little unclear.  I'd suggest that aviation compasses are typically not counterweighted in the sense of having a one side of the compass card heavier than the other side, but are weighted in the sense of having the CG of the compass card as low as possible relative to the pivot point to prevent the compass card from tilting in reponse to magnetic dip.  Some hand-held compasses (Silva etc) are counterweighted in the former sense and do have different versions for N and S hemispheres.

( Note also that as long the compass is undergoing neither linear acceleration nor centripetal acceleration, allowing the needle to dip does not actually introduce a systematic error in reading, but in a Silva-type compass it would tend to make the needle rub on the housing, essentially increasing the apparent friction of the pivot point.)

Actually Jpe61's linked question raises a third possible source of acceleration error-- an off-center counterweight.  Note also that an off-center counterweight that was "correct" for a given dip angle in a 1-G situation would be "wrong" if we changed either the dip angle or the G-loading.  (But, aviation compasses aren't expected to work well at G-loadings significantly above or below 1 G anyway so -- that doesn't prove much.)

@quietflyer an off-center counterweight is exactly what I'm describing in my answer. Or at least it's what I'm trying to describe An off-center counterweight balancing the needle against the dip will produce acceleration errors. As for the hand held compasses, it seems weird for them to be counter weighed. The dip can be overcome simply by slightly tilting the compass. That is actually very intuitive, since you instinctively try to level the needle inside the casing.

Much the same logic applies here as in this related answer to a related question.  How much tilt of the compass card due to dip do you see when flying east or west on a linear course with no acceleration?  Very little.  This suggests that aviation compasses are designed so that the CG of the compass card is low enough, relative to the pivot point, that the compass card tends to stay oriented rather close to the "felt" up-down direction in the aircraft's reference frame (which in non-accelerating linear wings-level flight is also the actual up-down direction relative to the earth), regardless of the effect of magnetic dip. So the compass card tends not to react to magnetic dip by tilting to any significant degree.  This suggests that "explanation 1" plays only a minor role.
However, just like a "plumb bob" weight hanging from a string, the low CG of the compass card will cause it to tilt during linear acceleration or deceleration.  Once the card is tilted, if the magnetic field lines have non-zero dip in the region where the aircraft is flying, and the aircraft is not flying due magnetic south or magnetic north, the card can better align itself with the magnetic field lines by rotating about the pivot point.  "Explanation 2" accounts for most of the observed error during linear acceleration and deceleration.

As it was taught to me, the acceleration errors of the magnetic compass are caused by the counterweight on the compass needle. This counterweight is installed on the needle (or dial) to keep it level despite the magnetic dip you mentioned.
While the small counterweight keeps the needle/dial level, it also shifts the center of gravity of the needle/dial from the axle, making it react to horizontal acceleration in certain directions.


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Why does the aspect ratio of a wing become less important at supersonic speeds?
My understanding is that an increase in the aspect ratio (AR) of a supersonic wing won't increase the efficiency like it would for a subsonic wing.
If the subsonic induced drag is:
$C_{D_i} = (C_L^2)/(\pi\times AR\times \epsilon)$
And the supersonic lift wave drag for slender bodies* is:
$C_{DWL} = 2\times \frac{C_L^2}{\pi\times AR}$
They're both proportional to $\frac{1}{AR}$ so why is the aspect ratio considered insignificant at supersonic speeds? ------> It seems like I'm missing something here.
*Does this term only apply things like fuselages and nacelles and not wings? If so, what does it mean for it to have an aspect ratio?
Peter Kampf please help

I'm putting this in as a comment because I don't have all the math. One thing which occured when  initially breaking the sound barrier was that as the shockwave reached the tail control surfaces there was a loss of control. The boundary of the wave blocked the pressure differences which allow effective lift. The second is a quote from the Israeli F15 pilot who landed his plane missing the right wing. "If you fly fast enough you are like a rocket and don't need wings!" This of course requires an engine capable of thrust vectoring.

@Rowan, just look at the F-16. It's proof that even a lawn dart with a big enough engine can fly.

Isn't is less about the drag and more about stability in supersonic profiles?

@alex: Uhh - compare the F-16 to the F-18 and the lawn dart becomes an aerodynamically refined aircraft. It is all relative. McDonnell-Douglas even sold the poor aerodynamics of the F-18 as an advantage by saying that performance deteriorates much less with the same external load than it does for the F-16.

The two main reasons for wings with a subsonic leading edge are:

The lower lift coefficient in supersonic flight, and
the need to keep wing thickness down in order to minimize wave drag in supersonic flight.

Your observation is correct: The lift-related drag coefficient is proportional to the inverse of the aspect ratio, regardless of flight regime for slender bodies. In all cases, lift is created by accelerating air downwards, and efficiency is better when more air can be accelerated by less.
But another factor is the lift coefficient, the square of it to be precise. Again this influence on drag is equal for sub- and supersonic flight, but the magnitude of the lift coefficient isn't. Take the XB-70, for example (source):

Cruise lift coefficient:  0.1 to 0.13, $c_L^2$ = 0.01 to 0.0169
Takeoff lift coefficient: 1.3 to 0.73, $c_L^2$ = 1.69 to 0.533
Landing lift coefficient: 0.626, $c_L^2$ = 0.392

Even at the highest supersonic cruise lift coefficient, the approximated lift-dependent drag coefficient is only 4.3% of what it is at landing speed. Supersonic drag is dominated by pressure and friction, and the cost of lift creation is dwarfed by the other sources of drag. Or let's look at another aircraft designed for sustained supersonic flight, or more specific, it's flight envelope (source):

The aircraft operates in a narrow band of dynamic pressures, trading density for speed to stay within a corridor of 310 KIAS to 450 KIAS when flying at supersonic speed. Here the lift coefficient is roughly between 0.1 and 0.3 (depending on aircraft mass and load factor) and again much lower than at low speed, where minimum speed drops to 145 KIAS.
Note that the lift wave drag formula for slender bodies is only strictly true for $AR$ approaching zero. At higher aspect ratios and especially with a supersonic leading edge this relationship is diminished. In supersonic flow the aspect ratio only comes into play by influencing the part of the wing that is within the Mach cone of the wingtip. In other words: The supersonic flow around a slender body is everywhere influenced by tip effects, whereas the inner and forward part of a higher aspect ratio wing experiences two-dimensional flow, being blissfully unaware of lateral limitations to wingspan.
Now to the structural factor: Wave drag has two components, one from lift and the other from the displacement effect. Pushing air out of the way at supersonic speed must be avoided as much as possible, because it is doubly punished: Surfaces facing in flight direction experience higher pressure than ambient, and those facing backwards experience lower pressure (i.e. suction). In all, zero-lift wave drag increases with the square of the thickness ratio, so there is no space in supersonic wings for the beefy spar a high aspect ratio wing would require.


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What do the tail numbers on aircraft mean?
Could someone please tell me the meaning of the alphanumeric numbers on aircraft's tails?
What exactly does the SB 023 mean for this Indian Sukhoi-30 MKI aircraft?

Are you interested only in military aircraft, or all aircraft?

All military aircraft seem to have these identifying tail labels, but the derivation of the symbols varies widely from country to country and even service to service. A related and more interesting question might be "What is the purpose of the alphanumeric numbers on aircraft's tails?"

I am interested mainly on military aircraft's....!

Naming tails after exceptional air warriors is an old Indian Air Force tradition. E.g. the Indian Air Force named the first Rafale trainer aircraft RB 008, in a tribute to Air Marshal Rakesh Bhaduria, who played a very critical role in the procurement of the fighter jets, and led tough negotiations for the 36 French fighter jets.
Trainer aside, the tail number of the operational Rafale fighters carries the BS series - BS 001 onwards - named after ex-IAF chief Air Chief Marshal B S Dhanoa.
KH - the tail number series for LCA HAL Tejas is named after scientist Dr Kota Harinarayana, programme director and chief designer of the indigenous light fighter, while the HAL HTT-40 basic turboprop trainer has ‘TSR’ in honour of the then C/MD of HAL T. Suvarna Raju.
Similarly, IAF MiG 27s carried TS series, named after a former officer of the IAF, and SB for Sukhoi 30s stands for two young Wing Commanders whose names were not disclosed. It could also possibly be named after Sqn Ldr Surya Bir Singh who won the Vayu Sena Medal for exceptional gallantry.
Interestingly, the C17 transport aircraft of the IAF carries CB after the former IAF chief NAK Browne, whose call sign was Charlie Browne.
The Tail Number is given by the Operations Branch of the IAF and then the series are allotted by the Maintenance Branch. These are nuggets which many don’t know, and there is no fixed parameter for tail numbers.
Earlier in IAF history, Hunters sported ‘BA’,the Canberra ‘IF’, the Gnat ‘IE’ but thencame the Russian-origin MiG-21s which just had the alphabet ‘C’, while the Sukhoi Su-7s carried ‘B’.
In the 1980-90s came various versions of the Jaguar (‘JS’; ‘JT’; ‘JM’), the Mirage 2000s were ‘KF’ ; ‘KT’, the MiG-29 ‘KB’, the swing wing MiG-23s were ‘SM’, MiG27s were ‘TS’ and the Sukhoi Su30MKIs ‘SB’, even though all aircraft
tail numbers are nowadays being airbrushed off in official IAF releases.

This is very interesting! Do you have some sources you could reference to back it up. Otherwise, it could be considered "some guy on the internet says...".


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Was the miracle on the Hudson saved exclusively by the APU?
In the film Sully, (staring Tom Hanks) the APU is switched on straight after both engines rollback, why was this step necessary; Airbus said this was the "15th thing" to do on the checklist.
How or why did this step in particular prevent the loss of lives of over 100 people?

Related: https://aviation.stackexchange.com/q/37700/2407

Yes, the solution to start APU was important.
The ditching procedure directs the use of maximum available slats and flaps for the final approach and touchdown (source, chapter 10.3). This is not possible without APU, as Airbus A320-214 cannot move flaps if only powered by RAT (only blue hydraulic line, same source, chapter 9.3). The running APU adds the green hydraulic line that can also move flaps.
Hence, without running APU, the optimal ditching procedure is not possible; that gives less chances for success.

+1 This is the most relevant answer. Without APU many useful/important systems would be down. Also, I assume the APU allowed them to continue flying in normal law, rather than reverting to direct law.

@Thomas -- you are indeed correct that starting the APU kept them in normal law (instead of alternate law) which meant that Sully had the full benefit of the Airbus FBW protections

Yes good point.

But he selected flaps 2, not flaps 3, because he was concerned about the drag at flaps 3...

To prove that the APU start was important, you have to not only show that the APU enabled flap extension, but also that flap extension was important. Saying that flap extension was 'optimal' may only mean that success chances with flaps is very, very slightly higher. (And given that operator recommendations are for a variety of situations you have to show that the optimality is true for these circumstances.).

Why does the RAT power only the hydraulic system that doesn't power the flaps, given that the flaps are so important for softening a no-engines landing?

I think the answer could be edited. Only flaps 2 was selected, thus not even slats were extended, which would have allowed for even lower airspeed. The most important factor after making the correct decision to land in the Hudson in the first place, was in my opinion, still having normal law. As demonstrated already in the 80s, over 20 years earlier, you can successfully crash land an A320 in a forest without significant injuries from the impact when just pulling back on the stick and achieving the α-FLOOR or α MAX speed at touch down / impact. Without full hydraulics it's not so easy anymore.

Sorry, this is incorrect. The APU does not pressurize the A320 hydraulic system, so it had no effect on the usage of flaps.

https://www.aviationmatters.co/airbus-a320-hydraulic-system/ A320 hydraulic system has electric pumps for Yellow and Blue lines and PTU between Yellow and Green so it is enough that APU produces  https://320pilots.com/air-conditioning/ electricity.

The Yellow system can be pressurized by the electric pump, and might pressurize the Green system via the PTU if done in such a way, but that's more of a theoretical question. It did not happen in 1549's accident, it does not appear in any checklists or emergency procedures involving the A320 I'm familiar with, and it is not mentioned in the NTSB report. It would also require that the electric pump be turned on after APU start, which it was not, and for the system to have depressurized in the first place, which it did not. And the air conditioning packs are unrelated to the HYD system entirely.

(Pt 2) I challenge you to provide an air carrier QRH or FCOM procedure dealing with dual green/yellow HYD failure that indicates starting the APU, charging the yellow system via the yellow electric pump, then pressurizing green via the PTU. Again, it's theoretically possible - I'd certainly try it, given enough time - but it's such a remote consideration and possibility that even in the aftermath of revised procedures following 1549, it isn't added. It isn't taught at any of the three A320 carriers I've trained at. Moreover, it was not done in 1549's accident - it's just factually untrue.

@Thomas Correct but, ironically and as Mentour Pilot pointed out, that meant that alpha floor protection (stall prevention) remained enabled all the way to the end, which lead to the aircraft not flaring nearly as much as Sully wanted (according to his recorded control inputs) on ditching. So switching to direct law immediately before would have probably reduced the number of injuries. The damage to the airframe is not really relevant because the aircraft was purchased by a museum, lol. Given the immense workload such an oversight can of course happen easily.

The engines provide electrical and hydraulic power to the systems that control the flight path of the aircraft: elevator, aileron etc. If all power is lost, a Ram Air Turbine (RAT) is deployed which spins up and powers the systems from the airspeed. However, this creates extra drag.
The RAT deploys automatically upon loss of AC bus 1 and AC bus 2. This takes a while after engine fail since the engines spool down - if APU is switched on before the buses are detected down, the APU then powers the AC buses and the AC pumps that drive the Blue and Yellow hydraulic systems. Below 125 knots the RAT stalls.
The RAT was found deployed in the accident report. It has an automatic blade pitch adjust system that retains constant speed under load - if the RAT is deployed and needs to provide the backup power for Blue hydraulic and AC electrical, it extracts energy from the air stream and provides extra drag. If the APU provides power for electrical and hydraulic systems, the RAT windmills and there is less drag.
The A320 had just taken off and did not have much altitude yet. Aircraft without engine power can glide - the distance they can cover while gliding depends on the starting altitude of course, but also on Lift over Drag: the lower the drag, the further the aircraft can glide. Switching on the APU provided them with extra gliding distance due to less drag - although that may not have been the captain's main objective, it may have been an attempt at engine start.
They needed all the distance they could get, and all the clearheadedness that could be provided. And that is what the pilots delivered. Switching on the APU bought them time, and more choice in landing the plane at a suitable spot. The situation was not exclusively saved by switching on the APU, it was a factor in a chain of events where everything needed to be right - and it was.

You mean that switching on the APU allowed to retract the RAT ? It's kind of implied by your answer but it would be nice to write it down.

Have amended answer.

Still unclear - how did switching on the APU "buy them time" exactly? Less drag? Better electrics/hydraulics than the RAT would have provided? Slight thrust? Other?  While the APU would give them a shot at restarting a motor, that isn't how things played out, so I don't follow your statement.

The RAT deploys automatically upon loss of AC bus 1 and AC bus 2. This takes a while after engine fail since the engines spool down - if APU is switched on before the buses are detected down, the APU then powers the AC buses and the AC pumps that drive the Blue and Yellow hydraulic systems. Below 125 knots the RAT stalls.

@RalphJ Indeed, less drag so longer glide distance, and a shot at restarting an engine.

It appears the RAT extension logic is different between A320 and A330. So preventing extension was possible on A320 only?

I haven't worked on the A330, don't know what the N3 stands for in the N3 < 50%. Presumably there is some time as well before both engines spool down.

Have amended the answer again, thx.

I understand what you're saying. But it does sound odd to use "windmills" to mean "does not extract energy from the air stream" -- that's what a windmill does!

Can we say anything quantitative about what the reduced RAT drag did for the aircraft's overall L/D ratio, though?

@RalphJ, the drag was all but irrelevant. The RAT has 5 kW electric power. I don't know the value for hydraulic power, but it will be comparable. Let's be generous and say they saved 20 kW. $P = mgv_v$ They had at least 60 t. That gives at most 7 ft/min. Less in practice, since I intentionally overestimated. That negligible compared to the over 1000 ft/min glide. Moreover, once they concluded they don't have enough energy to reach the closest runway, they were not limited by the energy anyway. What did help, though, was the ability to control flaps as mentioned in h22's answer.

The main advantage of the APU over the RAT is more power, not less drag.

From the accident report: Contributing to the survivability of the accident was...  (3) the performance of the cabin crewmembers while expediting the evacuation of the airplane; and (4) the proximity of the emergency responders to the accident site and their immediate and appropriate response to the accident. Who knows what the extra distance meant?

@Thomas yes all three hydraulic systems were powered at impact and all flight control surfaces were active. As opposed to RAT which only powers Blue hydraulic, which only powers half of elevator & aileron surfaces. He had a fully controllable aircraft. More power - less drag - some thrust from the APU. Every bit helps.

There's several inaccuracies in this answer and the comments. The advantage of the APU was that it provided a greater amount of electrical power than the RAT can provide. The APU provides no thrust, and the drag from the RAT is negligible. The use of the APU had no direct impact on the L/D ratio of the aircraft.

No, the "Miracle on the Hudson" flight was NOT saved exclusively by the APU.
Airline flights are routinely dispatched with a non-operable APU. It is not needed or required to complete a safe flight.
On the Boeing B777 starting the APU(if avaialable) is number 5 in the checklist for dual engine failure.

In my opinion the starting of the APU had very little to do with the success of the water landing and survival of all the passengers and crew.
Sully is on record as saying he started the APU to provide an addition electrical source:  Sully Speaks Out
I had taken by memory the first two remedial actions that would help us the most, that we would later get to on the checklist over a minute later, over a third of the way through the remaining flight time, and that was to turn on the engine ignition so the engines would recover if they could (it turned out they were irreparably damaged), and I started the airplane’s auxiliary power unit, the APU, to provide a backup source of electrical power. Since this is a fly-by-wire airplane and there’s no longer a direct mechanical connection with the flight controls, instead you have electrical impulses that are fed through computers that then move actuators.
In this particular case the APU may have allowed a slightly longer glide, but that would just have meant touching down on the water slightly upstream from where they did touch down.
Map Source: CNN Map

No, the "Miracle on the Hudson" flight was NOT saved exclusively by the APU All three answers should have made that the "bottom line up front" as you did.

I realize this is an old question, but I came across it while looking for something else and it bugs me that the existing answers are conjecture and incorrect (though I'm sure well intentioned). So, in case anybody else comes looking some day...
The advantage of the APU in this situation was to provide greater power. When the electrical busses on an A320 lose power, a small windmilling propeller called the Ram Air Turbine (RAT) deploys automatically when the aircraft is above 100 knots. The RAT provides a minimum of power to support crucial aircraft systems, and it also powers one of the three hydraulic systems. The APU, on the other hand, can power the entire aircraft.
As far as the electrical side of things goes, this allowed the aircraft to remain in Normal Law. Airbus aircraft use a set of flight control laws that determine certain protections and operating margins. In this case, it allowed the aircraft to remain a safe margin above stall speed for as long as possible. Without Normal Law protection, which would've been lost if operating only on the RAT, some of those protections would be lost. In other words, it allowed greater control protections to help with a smoother landing.
On the hydraulic side, it gets a little complicated. The three hydraulic systems are variously powered by the engines or electrical supply. The RAT can only power one of these three systems (blue). With this single system powered, some controls would operate normally, some at half speed, and some not at all.  The APU can directly power the blue system, same as the RAT, and could power the other two systems depending on some switch settings. The US Air crew didn't have time to change those settings, but as it turns out, they didn't need to. The hydraulic systems weren't leaking, and with the short time between the birdstrikes and forced landing, enough pressure existed to operate the control surfaces normally.
So, short answer: the APU was helpful in that it provided electric power, which enabled the crew to have better control of the aircraft. It didn't affect the hydraulics, which remained pressurized, and it did not add thrust. It was one of many things that the crew did right, but not the only one. How much of an impact it made it difficult to say, but in an emergency scenario, you'll take all the help you can get.
Edit to add: Another user asked for some documentation regarding the APU and hydraulic systems. All aircraft have flight manuals, including airliners, though the more complex they get, the longer the manuals. At the airliner level and on the pilot side, these are usually distributed as a Flight Crew Operating Manual, or FCOM. FCOMs are not typically publicly available, as some of the information inside is proprietary or otherwise sensitive.
With that said, you can find nearly anything via Google, and there are a number of Airbus A320 FCOMs floating around online. Here's a link to a website featuring some generic A320 FCOM sections which are not specific to any particular air carrier. These are typically issued by Airbus for companies to customize or for training purposes. Here's a specific link to the APU section and one to the Hydraulics section (these two are PDFs). Note that in the first, hydraulics are not mentioned among the APU's functions, and in the latter, the APU is not mentioned among the sources of power for the hydraulics system.
Finally, here's a picture of the overhead panel on the A320. One of the nice things about the A320 flight deck is that there is a basic systems diagram on some of the panels, acting as a quick reference for major components of the system. Note how Green and Yellow systems show engine pumps, Blue and Yellow show electric pumps, the PTU connects Green and Yellow, and the RAT can also power the Blue system. Were hydraulic power available from the APU, it would be shown here (and would require a pushbutton), consistent with Airbus design philosophy.

The chosen answer cites the official NTBS report but somehow your answer contradicts that answer: can you also support your answer with some official document? If so, then your answer becomes in my opinion the most complete (and correct)

@sophit, sure. Need to split this into two comments. Part 1 - The answer you mention cites the NTSB report regarding using both slats and flaps (10.3) and that flaps will not operate solely on blue hydraulic power (9.3). The following sentence is where it goes wrong: the incorrect statement that the APU powers hydraulics. This is not in the NTSB report or in any Airbus FCOM (Flight Crew Operating Manual), as the APU and hydraulic systems are not connected. FCOMs are carrier-specific and not typically public material, but I've added a link to a generic Airbus FCOM to my answer.

Part 2 - The same answer also ignores a statement in the report a few lines later, also in section 9.3: “Preliminary information obtained from the DFDR indicated that none of the three (green, blue, or yellow) hydraulic systems indicated low pressure… [indicating] that the selection of flaps 2 in the accident scenario resulted in slats 22 and flaps 15.” Even if they were correct regarding the APU’s purpose, the hydraulics maintained pressure long enough to complete slat/flap configuration. Absent a leak, the systems don’t immediately lose all air pressure.

Perfect, thanks, +1

Jep, I think it was very beneficial to the outcome of this event. However I think that in addition to the reasons stated here, the APU also prevented Sully from stalling the airplane.
If I remember correctly, if all electrical power is lost/provided by the RAT, the airplane goes to "direct law" mode. This means, that all inputs by the pilots are translated directly into proportional movements of the flight control surfaces. The systems that usually keep the pilot from getting the plane into a unrecoverable situation are deactivated. As Sully pulled the plane up just before touchdown in order to execute a flare, he would have stalled the airplane, had the fly-by-wire system not prevented him from doing so. As this system would not have been active if he had not switched on the APU, he would have dropped into the water like a stone.
So yes, the APU saved the day, by reducing drag, burning a tiny bit of weight of, moving the flaps, and keeping Sully from stalling.

For the A320, emergency electrical configuration (RAT/flight on batteries only) with the gear up results in alternate law. When the gear is extended, the aircraft goes into direct law.

sorry, did not research coerrectly. Alternate law 1/2 don't have AOA protection either, so the rest is correct. I will edit my answer and mention you, ok?

Sounds fine to me.

APU was absolutely necessary to provide power to the electrical hydraulic pumps as the milling effect was much reduced by the damaged fan and other items in the engines, further the RAT alone when not stalling gives a slow rate to the surfaces


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Is this contrail from an airplane or a rocket?
A relative of mine showed me the following picture that she took on the 11th of April, around 8 to 8:30 pm in the Netherlands somewhere near "Leiden" pretty close to Schiphol airport (unfortunately I do not know the direction she was looking at, or the exact location, nor does she remember).

This relative thinks that what is shown in this picture is a rocket ascending due to the steep vertical incline. I however doubt this and rather believe it is a plane flying towards the photographer. The illusion of it ascending steeply being caused by the shadow of the earth falling close to the "plane" while the distant contrail is still in sunlight due to the height at which a plane flies, creating a fake depth effect.
What makes me doubt my theory however is the (to me seemingly) irregularly shaped contrail at the end which look like the plane did some weird zigzagging (or a very strange wind blowing it like that), and it seemingly "just appearing" out of nowhere
Now we both don't know a lot about airplanes, so I was wondering if by looking at these contrails it is possible to confirm or deny that this is caused by an airplane.
Looking at the flightradar for that moment there appears to only have been one plane in the area, and even that appears to be quite far out. Could this plane possibly have left those contrails?

The direction of the photo should be west – the setting sun is just right of the right edge. While the jagged contrail really resembles that of rockets, the light on its more remote part clearly shows that is is from an aircraft. A rocket contrail in that condition would become brighter the higher you go, and this becomes darker as the aircraft moves eastwards.

Discounting the obvious wind distortion, it'd be even more amazing if an ascending rocket followed the path of that contrail than if an airplane did. They don't have as much time to zip about when leaving a trail. Enough g-forces and you kill everyone on board.

Obligarory XKCD: https://xkcd.com/1677/

The zig-zagging is caused by winds.

@HotLicks: nope - that's the rocket flying an evasive course to dodge the flying saucers which patrol Earth-space to prevent us from resupplying our advance base on Mars. Oh, wait - I wasn't supposed to reveal@#)(&@)#%^(@#$@#()&)$(#@!NO CARRIER

I am pretty sure it is an aircraft.
When aircraft fly straight towards you they can appear like rockets. The contrail is distorted by winds in the upper airspace. Yesterday there were a lot of cirrus fibratus and cirrus uncinus in the upper atmosphere (at least above Frankfurt, and we are in the same weather system at the moment), and when aircraft fly through them the contrails typically wobble like in your picture.

Now, which aircraft was it?
The aircraft visible on the picture is not flying in sunlight anymore, but most of its contrail still is. It also just made a turn.
On the eleventh of April, sunset in Leiden was at 20:33 local time (18:33 UTC), setting in a bearing of 285 degrees.
Flightradar24 show this DHL Airbus A330 descending from FL 390. I would estimate that this picture was taken at 20:36 local time, perhaps a bit later. The turn is clearly visible on the picture and on FR24. The contrail matches that of a large twin engine. (my first guess was a 777, an A330 has a similar contrail. It is definitely not a 747).

A comparison picture created by ymb1 shows the shadow/lighted parts of the contrail highlighted, contrasted with the terminator on the map.

Amazing information about those winds, clearing up our biggest question! (Awarding you the checkmark for that)

@Remy Dankjewel. Ik heb ook nog even uitgezocht welk vliegtuig het was. Groeten, DL

"Yesterday there were a lot of cirrus fibratus and cirrus uncinus in the upper atmosphere (at least above Frankfurt, and we are in the same weather system at the moment), and when aircraft fly through them the contrails typically wobble like in your picture."-- I would suggest that these types of clouds are indicative of high moisture in upper atmosphere, allowing contrails to linger for a long time, allowing wind shears to distort them-- as opposed to being directly indicative of wind shear.  I may be wrong, perhaps in some way they do directly indicate wind shear as well.

@quietflyer These clouds are indicative of wind shear in the upper atmosphere. See wikipedia about wind shear. Cirrus uncinus ice crystal plumes showing high level wind shear, with changes in wind speed and direction.

Thank you for that bit of education.

Going from "is this a contrail from an aircarft?" to identifying the exact flight and time - I love this site!

Aw man. The information age is making UFO's hard to come by. : )

@DeltaLima Amazing analysis.  Can you provide some insight into what it is about this contrail that indicates a large twin, as opposed to a four engined aircraft (ex 747, A380)?  My completely uneducated eye doesn't know what to look for.

@candied_orange Reminds me of how 4chan managed to track down the location of a livestreaming camera in a field by noting down the images of aircraft flying through the sky.

@AleksG And now lets get the Pilot to describe what he or she remembers about that turn:)

That is a contrail from an airplane, not a rocket.
The wideness of the trail at the bottom part of the photo makes it clear that it has been lingering for some amount of time.  The distortion is clearly due to the effect of wind shear acting on the lingering trail, not due to radical maneuvers.
Rocket engines burn for a relatively short time, and rockets have steeply climbing trajectories.  If that dot at the end were an actual rocket, the portion of the trail at the bottom of the photo would be only a few minutes old at most, which is unlikely to be enough time for changes in wind velocity with altitude to produce that much distortion in the lingering contrail.  Also, it is difficult to think of a scenario where a rocket's steeply climbing exhaust trail would be sunlit on the bottom and shaded on the top.  The opposite would normally be true-- as in this incredible photo from the Space Shuttle mission STS-98.

Image from NASA
It is not that uncommon for an airplane's basically horizontal contrail to look like this.  The distant portion, which is at the bottom of the photo, may be well over five minutes old, allowing wind shears more time to distort the trail-- although in truth this much distortion would be more common in a case where the trail had been lingering for several tens of minutes, with the airplane itself long gone.  But in fact I have taken a photo much like this, with the airplane still approaching, and joked with somebody standing beside me about the "crazy maneuvers" that it looked like the aircraft had undergone.  Of course when the aircraft passed overhead it was clearly just a commercial airliner flying in a straight line.
The sun is clearly located toward the lower right of the picture--see the diagonal shadow streaking up and left from one of the sharp "bights" in the trail.  Note that a similar diagonal shadow line is faintly visible running up and left from the very bottom right corner of the photo.  Follow both lines down and right and see that they converge at a point below and right of the lower right edge of the photo-- the sun is clearly below the horizon as seen by the camera.  The sun is also clearly below the horizon as seen from the aircraft's current position-- as suggested by another answer, the aircraft has clearly flown past the terminator of the sunrise or sunset, passing into the shadow of the earth itself. This would be very unlikely with the near-vertical trajectory of a rocket.  In the rare event that it did happen, the photo would look quite different than this-- you would see the trail rising up steeply and then arcing over toward a more horizontal trajectory in a way that just doesn't fit what we see in this photo.
While other answers have pinned down with confidence the exact time and location of the flight, note that the principles applied in this answer could be applied to any similar photo without knowing any more information.
Let's close out this answer with a few more notes on the spectacular photo from the launch of STS-98.  This mission was launched just after sunset.  Although the trajectory is arcing over the toward the east, the extreme altitude gained in the vertical part of the trajectory has caused the vehicle to pass from shadow into sunlight, rather than vice versa.  Note the long shadow, cast through the still-sunlit upper atmosphere by part of the exhaust plume itself, ending at the anti-solar point just below the moon, which is about half a day from being exactly full.  The shadow is made more visible by the fact that that much of the dust-and-moisture-filled lower atmosphere between the observer and the shadow is already shaded by the earth itself, reducing glare.
Someone standing 5 miles away would have seen something quite different-- the shadow line might not have been visible at all, or a different part of the trail might have been aligned to cast a visible shadow line.   Of course, the shadow (if visible) would still slant through the sky at the same angle, and would still end at the distant anti-solar point, just below the almost-full moon.
For a contrail or exhaust plume to cast a visible shadow through the atmosphere like this, a significant segment of the shadow must be directly aligned with the observer or camera.  This can only happen when a significant segment of the contrail itself is directly aligned with the sun as seen by the observer or camera.  In other words an imaginary line extended from that linear segment of the contrail must appear to pass directly through the sun as seen by the observer or camera.  The easiest way to catch a glimpse of this phenomena in action is watch for an instance where a long, linear contrail is just upwind of the sun as seen from your position.  As the contrail drifts through the sun and then downwind of the sun, its aerial shadow will appear and then vanish again.  Then another observer standing further downwind will have a chance to observe the same thing.  The aerial shadow from a contrail or exhaust plume is like a pane of window glass-- it is invisible unless you are looking at a significant length of it directly edge-on.

A rocket going up would indeed have its "head" lit by the sun while its "tail" would be in the shadow, as greatly illustrated in this photo by NASA

@Ruslan -- amazing-- added links and description to answer

That is just an illusion. The aircraft is flying straight and level and the contrail behind it is being distorted by the winds aloft.

authors: Bob Jarvis - Слава Україні


text:
Is a catapult takeoff safer than a normal takeoff?
With all the increasing interest lately around on Urban Air Mobility UAM, AAM, VTOL and STOL Short Takeoff and Landing, I thought if the aircraft carrier catapult takeoff functionality could be part of these new VTOL/STOL urban airport designs that are being conceptualized these days.
I thought if as part of reducing the runway length requirements a catapult could be installed. So I wondered, safety always being the no 1 requirement if, are there more accidents on catapults takeoffs than with normal takeoff and if it would make sense using a catapult for UAM STOL?

Interestingly, the first use of a catapult to launch an aircraft was with the Wright Flyer...  https://www.wright-brothers.org/History_Wing/Wright_Story/Inventing_the_Airplane/Little_More_Oomph/Wright_Catapult.htm

On a carrier, a catapult takeoff is slightly safer than a ski jump takeoff. The catapult gives you extra energy, not much, about an extra 4 seconds of flight. With margins as tight as they are on a carrier takeoff, these four seconds can make a life or death difference.
A carrier takeoff cannot be aborted once the catapult goes off. A plunge into the drink is invariably fatal. The only way to abort is to eject and lose the plane. It happens, and naval aviation hull loss rates are two orders of magnitude higher than commercial.
Things are different on dry land. There, the takeoff roll is used to check if the aircraft is behaving normally, and if it isn't, there are two decision speeds: ~80 kts to reject for any issue, and V1 at 120-160 for critical problems. Aborting problem takeoffs is vital for achieving one-in-a-million or better crash rate expected of commercial flight.
The reasons catapults aren't and are not likely to ever be used for passenger flight on land are exactly the same as why more affordable ski jump ramps also aren't used - they lack this margin of safety. To restore it, you'd need to bring the runway up to nearly the same balanced field length as without a catapult.
However, assisted takeoff isn't entirely unused on land. It's common for gliders to be winch-launched, which is basically a low-tech catapult, but can be aborted mid-launch, and it's common to catapult launch UAVs. These risks are mitigated or accepted: for gliders, the engine is optional and it's a sport, and drones are just hardware.
Also, if a runway's too short for a conventional takeoff, is too short for a safe landing. You want a margin before and after the perfect touchdown point, and enough length for a touch-and-go if you touch down too late or too softly. Carriers use arrestor wires or maneuvers like SRVL, both with high deceleration and little margin for error. Naval aviators have ejection seats, and it's still a high-risk occupation even by military standards.
While urban air mobility is likely to remain a pipe dream for a long time, its possible use of assisted takeoff hinges on whether a higher loss rate is acceptable. For delivery drones it's "yes", for enthusiasts "possibly", but for common passengers, an almost certain "no". Even if ejection seats were fitted, they're not a 100% save and too dangerous for the general public.

The other significant reason for not using catapults is that the runway is also needed for landing, and at a similar length.

@MikeB Well, you could cable-trap the STOLs. Just make sure to only serve oatmeal aboard.

@Therac I concur with MikeB. Doesn't a landing with arresting gear like a tail hook need unusual skill, and isn't it unusually prone to failure? When you start you have all the time in the world to position the plane and make sure everything is attached and ready. When you land, not so much.

@Peter-ReinstateMonica You are right, these landings take skill to nail. But I'd expect 2050s automation to help. Also, in a flying aircraft, you have more options, like diverting to a real airport, or a barricade traps as a last resort.

Winch launching doesn't have the same dangers as catapults, because you can abort a winch launch at any time. Very rarely manned gliders can be bungee launched, which is essentially a catapult. https://www.a-e-g.org.uk/the-bungee.html Happened to me in an air cadet  T38 Grasshopper in the seventies.

Another feature of carrier operations is that wouldn't usually be anything past the end of the carrier's runway that one would worry about damaging.  In most scenaios where a land-based runway would need to be unusually short, a failed takeoff or landing would jeopardize more than just the plane and pilot.

A catapult needs both more infrastructure on the ground, and a much stronger airframe.
Both of those things mean much more money.
Catapults work on a Navy carrier, because there is literally no other option. Runway length is rarely that much of a limiting factor with an airport.

Thanks WPNSGuy, I want to focus the question on safety and practicality in the aviation sense(maybe ignoring the obvious costs, though actually that could be argued because you could compare the cost of a catapult system vs cost of bigger land). Please note that UAM is aiming for really high dense cities, some of these airports are to be constructed on very tight spaces, some even on rooftops.

Given a rooftop catapult takeoff...where are you going to land? Arresting hook capture on that rooftop is unfriendly to regular passengers.

@Gabe: Short takeoff / vertical landing is a thing some military aircraft do.  IDK how plausible it would be for civilian craft (weight, power-to-weight ratio at landing weight, extra flying skills needed(?), safety of basically hovering - if anything fails and you don't have an ejection seat, you're falling from too low to gain speed and glide, but a hundred or two hundred feet are still a long way).  If viable, that could let you land on something the size of a catapult strip without a tailhook sudden stop.

In short
Between the title and the body of your post, you ask disconnected questions about catapult safety and catapult use in urban air mobility.

A takeoff with a catapult is not safer than regular takeoff.

UAM projects don't rely on landing strips, they are based on vertical takeoff and landing, and electric rotorcraft.

Catapults are not helpful in vertical takeoff.

1. Catapult safety

Is a catapult takeoff safer than a normal takeoff?

No. If a catapult is needed to take off, it's because the aircraft is not able to safely reach the minimum speed using the available runway length. Using a catapult is not a way of adding safety, it's rather a way of making possible what is otherwise impossible. This impossible-made-possible comes with its own problems and risks.
Problems appear when the catapult cannot deliver the expected acceleration, when it blocks abruptly or when the aircraft cannot be uncoupled from the catapult shuttle, etc. These are additional risks compared to a regular takeoff.
Limiting these risks requires a reliable design, frequent inspections and continuous maintenance, meaning additional costs for the operator and for the customers. This is clearly not compatible with the UAM economic model.

2. Catapult and UAM projects

[Whether] it would make sense using a catapult for UAM STOL.

I would say it doesn't make sense. UAM projects don't rely on horizontal takeoff or landing, nor on catapults. UAM projects I'm aware of use electric rotorcraft taking off and landing vertically. Such aircraft known as eVTOL, are for example already used by Volocopter, or planned by Airbus.
They take off from "vertiports" (low cost heliports) and rooftops in city centers. In case of heavy wind, a vertical catapult adapted to eVTOL might provide a last resort way to take off, but this advantage is lost for landing. In this case, an eVTOL aircraft just stay on the ground.
UAM is "urban" mobility. In many parts of the world, e.g. in Europe, it's difficult to convince people to accept more noise or pollutants, so the usual argument to get authorization for eVTOL aircraft in urban areas is they can be used to quickly deliver medical supplies.
However this pretext doesn't change the actual business model which is to provide air taxis to people in a hurry, or pizzas and other cargo to the mass. These actual applications are intended to compete with land vehicles, not to save lives, so their cost must be low. Hence they cannot depend on usual aircraft landing strips, even short, they must be eVTOL.
If a catapult was ever required for eVTOL, it would have to be vertical, with some inconvenience:

Electric rotors would need to be adapted to bear the acceleration and allowed to rotate faster than their motors.

Taxi passengers would feel a negative acceleration, in the body axis (from head to feet), which is more difficult to stand than an orthogonal acceleration, and at the same time the passengers can't be lying on their back like astronauts.

I would imagine a eVTOL catapult could still be horizontal: helping the helicopter (which a eVTOL is) out of the danger zone without going through HOGE: https://aviation.stackexchange.com/a/54637/30268


text:
What is the least turbulent altitude under 10,000 feet ASL that planes usually fly?
Statistically speaking, at what altitude under 10k feet there are less chances for turbulence during a typical flight?
Personal experience is also welcomed.

This will no doubt be highly dependent on location.

And on conditions on a particular day. A smooth altitude today may be bumpy tomorrow, and today's rough altitude may be tomorrow's best ride.

Should this question's title be rephrased to either use AGL or MSL?

Upon further research I found a useful article which references an NCAR study and answers this question.
"Considering both in-cloud and out-of-cloud turbulence, flights between 8,000 and 12,000 feet will allow for the smoothest ride, on average" Turbulence Versus Altitude

What does this chart's horizontal axis measure?  And is the vertical axis AGL or MSL?  The linked article doesn't say.  Neither does it name the actual study, so we can't see for ourselves.

I'm also curious how much of this is reporting bias, or what the actual study might have done, if anything, to account for that. You could look at the graph and (I guess, depending on what the axes mean) conclude that 35,000ft is the most turbulent place to be, but surely there are more PIREPs reporting turbulence around FL350 because that's around where airliners fly and they're more likely to report bad ride conditions.

@ZachLipton yes, those are PIREPs, I've updated the NCAR link to the actual pdf file.

There are 3 main sources of turbulence:

Mechanical, from air flowing over the surface, like river rapids.
Convective from solar
heating of the surface (discounting other surface heat sources which are too minor to bother with).
Mechanical, from shearing of airflows going
in different directions above the surface.

Numbers 1. and 2. will be the vast majority of the turbulence you will encounter. All that is required is hilly terrain and winds in the case of 1., or sunshine in the case of 2.
Number 3 requires special conditions of air flows with sharply diverging directions and resulting shear effects, or "nocturnal jet streams" where a strong shear from a sudden change in velocity can exist at a few hundred feet over flat prairie topography.
Therefore, statistically, the vast majority of turbulence you'll encounter below 10000 feet originates at the surface.  The height above the surface will be a function of lapse rate and relative humidity in the case of convection (it generally stops at the cumulus cloud base) or the wind strength and surface contour in the case of mechanical turbulence.
Since it originates from the surface one way or another most of the time, the majority of turbulence declines with altitude, and therefore the least likely altitude, below 10000 feet, to encounter it, is 9,999 feet. (Note that in mountainous terrain, 10000 feet AGL may not be enough to get you clear of mechanical turbulence, but the concept is the same).
And in day to day flying, "higher is smoother" is obvious.

While the turbulence, including the thermally driven one, does "originate" at the surface (due to the surface heat/buoyancy flux), the maximum strenght is higher. The thermals are gaining velocity while accelerating in the boundary layer. The lapse rate is important, but mainly only to find out where the inversion is, the turbulence profiles are functions of w*, which is given by the inversion height and by the surface buoyancy flux.

But the perceived intensity declines with altitude when you fly through them as they spread out and the shear zone at the margin gets wider. IOW the bumps you feel from thermals gets softer and softer as you go up. When soaring, they get much easier to stay in.

Actually, the turbulent kinetic energy may have a maximum close to the ground, due to the horizontal velocity components. The variance of the vertical velocity component is what has a maximum at those 1/3 to 1/2. What you describe is quite possible. The effects on an airplane also depend on the length scales and time scales of the turbulence, not just on the variances only.

Since turbulence is many times caused by terrain, one might say 9999 feet.  However, turbulence can also be caused by passing fronts.  This is why METARs list wind speed and direction at various altitudes.
Flying near a wind shear interface can buy some fairly serious turbulence, along with some inclement weather.  Lower altitudes may be better after a cold front passes.

Most turbulence during the day will be thermally driven, not mechanically. In that case the maxium is not at the surface, but somewhere between 1/3 to 1/2 of z_i.

@VladimirFГероямслава that will depend alot on the prevailing wind and terrain, but, yes, on a calm day fair weather cumulus will pop up and dominate the local wind flow.  As for thermals "accelerating" we can certainly see that if there is condensation.  Let's hope Camille G expands on the graphs presented in one answer.

In the daytime, most turbulence in flattish terrain comes from atmospheric convection in the atmospheric boundary layer. The convective boundary layer can be cloud-free or cloud-topped (cumulus or stratocumulus).
If clouds are present, turbulence strongly increases inside them, due to turbulence generation by the buoyancy flux due to the released latent heat due to water vapour condensation. For stratocumulus it is quite a homogeneous layer, but for cumulus it si very concentrated to the cloud cores.
Above the boundary layer, we usually assume laminar flow without turbulence. That is not completely true of course, as internal gravity waves travel there and they could break, but that usually happens much higher, even in the stratosphere.
All this is for fair weather situations, mostly anticyclonic. When atmospheric fronts form, the clouds systems form with them and a lot of turbulence inside them. But in that case the height dependence will be complicated and and probably will differ case to case.
In general, the air above the boundary layer should be mostly turbulence-free. Statistically, the BL top will be up to 2-3 km in summer, less in winter.
During the night a stable boundary layer forms. It is much shallower, in hundreds of meters. Near its tops we often see a region of air velocities that are significantly higher than the geostrophic flow (supergeostrophic). This region is called a low-level jet. The shear there is associated with mechanical turbulence, but in general, the turbulence there will be much larger during the day in the convective boundary layer.
I repeat, above the typical boundary layer heights of 2-3 km the chance of turbulence should be quite similar. This may be affected by topography in mountainous regions and if you are considering an airport located in a deep mountain valley, you certainly can expect an elevated maximum of turbulence at the layer of the mountain ridges, but for flattish terrain it should be quite similar above the BL.

This question has no answer because for each altitude  < 10000, there is an altitude ′ such that  < ′ < 10000. Each such ′ is smoother than .
Delegate the tasks of getting from  to such a ′ exactly, measuring the actual turbulences, and getting the accurate statistics to the student pilots.

While this may be theoretically thrue, it's not very helpful as an answer :)

@Jpe61 The ability to delegate is undoubtefully a useful and HELPFUL skill. The original poster should exercise it! As well as thinking properly about a question before asking.

What exactly do you mean by delegating in this context? The fact that you have, probably just, learned to formulate stuff in a fancy matter does not make you eligible to come on ASE and be a smartass.

@Jpe61 The fact that you don't know the meanings of “under“ and “delegate” doesn't make you eligible to produce nonsensical comments and be a smartass. In fact, quite the opposite.

You're claiming that the least-turbulent altitude is always infinitesimally close to 10000 ft.  e.g. at any local minimum in turbulence below 10k, there's always a higher altitude with strictly less turbulence.  If you're already at or above a local minimum, climbing an infinitesimal amount will be slightly more turbulent, so the only way your statement could be true for all $a$ is if there's another smaller minimum higher up.  This contradicts all the other answers but you present no evidence, and your answer doesn't explain this practical implications of your invariant.

@PeterCordes If you are considered about practicality, show me where 10000 ft - , for a very small , is a local minimum.

I'm not one of your student pilots.  You show me; it's your answer making these claims.

@PeterCordes It's you who wanted “practical implications” (source: your comment). So go on and show us where on earth 10000 ft -  is PRACTICALLY both a local minimum AND less turbulent than 10000 ft. Feel free.

I'm pointing out that other answers have said the global minimum of turbulence isn't always 10000 ft -  for the [0, 10000 ft) range.  The slope infinitesimally close to 10000 ft or a local minimum near there aren't the problem with your answer.

@PeterCordes That's not what you originally said; you said “climbing an infinitesimal amount will be slightly more turbulent” [my emphasis]. I can live with “the global minimum of turbulence isn't always 10000 ft - ” [my emphasis], fair enough.

Your quote omits the condition from the sentence it's part of: If you're already at or above a local minimum  Come on dude, I didn't claim that's globally true about the physical world, I just stated the obvious about slopes on a graph on one side vs. the other of a local minimum, to explain my reasoning for why your answer is making that bogus claim about the world.

@PeterCordes Come on dude, I nowhere claimed that your statement would be globally true. Your comment is bogus simply for two reasons: first, you are boring, and second, you wanted practicality (which was not intended anyway), and you failed to show us where on earth 10000 ft -  is a local minimum for a very small .

Wow, 8 whole downvotes from the Aviation Police Squad! I'm impressed by your collective ability to misfire on the wrong target. Clearly, the question was the real culprit, but you folks were too busy flexing your downvoting muscles to notice. Keep on keeping on, heroes!

authors: Vladimir F Героям слава


text:
Does a carbon fiber fuselage suffer wear/fatigue from pressurization/depressurization cycles?
I've heard many times that one of the factors that causes wear and tear to the fuselage of airliners is the pressurization and depressurization from each flight.
From what I recall these were referring to aluminum fuselages.
Does this also apply to a carbon fiber fuselage? Is a carbon fiber fuselage affected in the same way aluminum is on each pressurization/depressurization cycle?

While the pressures were more extreme, Titan comes to mind.

Yes, it does.
CFRP's fatigue manifests through multiple mechanisms. It usually begins with matrix cracks, causing a loss of bonding, and grows through delamination, i.e. loss of adhesion between the layers of fiber fabric or tape.

Initially it was believed that CFRP has a fatigue limit, similar to carbon steel.
"Fatigue limit" means that, if loaded lightly enough, carbon steel will not suffer fatigue at all. That limit is quite high (just over 50%), and at design load for less-weight-sensitive applications (e.g. for ships it's 40%) it's usually within its fatigue limit. Steel in a ship can realistically experience a billion load cycles.

In comparison, any load on aluminum leads to a small amount of fatigue. High-strength, high-alloy steel used in aviation, e.g. maraging steel, also doesn't have a fatigue limit, but remains extremely fatigue-resistant.
Gigacycle tests, done in 2003-2005, have changed that opinion. Their results show that CFRP fatigue does continue past the previously-thought fatigue limit, even at loads as small as 30% of the static limit. So there's no known loading small enough that it won't eventually lead to failure.
When the term "fatigue limit" is used for CFRP today, it should be taken to refer to the engineering custom of using strength at 10^7 cycles, rather than a true limit. Note that gigacycle tests also question if it exists even for metals; rather, 10^7-10^9 just might be a flatter region.

You can find an analysis of factors that impact CFRP's fatigue life, as well as a review of the mechanisms. To get the full papers with all the graphs, one needs academic or other science paper access, though.
A notable finding is that the fatigue failure behavior of CFRP is highly non-linear. Its loss of strength is rapid for early intralaminar failure (within a layer), then slows down as the interlaminar cracks are expanding.
The mechanisms and the curve also differ a lot depending on how the composite is built up and what the loading is.

It's not so much that CFRP is unique in its fatigue resistance, as that aluminum is an outlier in being particularly vulnerable to and having its service life dominated by fatigue-related failure modes. Aside from regular fatigue, it can fail to stress corrosion. Steel, titanium, nickel, copper, and other structural metals are also subject to fatigue, but it's less dominant in their failure behavior.
To be specific, the minimum weight of a CFRP fuselage is mostly determined by impact and primary static loads. It has excellent tensile strength to resist internal pressure, but no plastic region at all, so where metals can bend to distribute the energy of impacts or concentrated stresses, carbon stands firm till it breaks. For most aluminum fuselages, it's low fatigue strength that sets their minimum weight.

TL;DR version: Yes, but carbon fiber has bigger problems than fatigue, while aluminum doesn't. That's why it's economical to use a lower pressure altitude for CFRP fuselages.

I wish there was a chance to give +2

I added my +1. this is a fabulous answer!

Plus 5 for this answer.

Astonishing from beginning to end

You should be able to click money!